1. Field of the Invention
The present invention relates generally to an active suspension system for an automotive vehicle. More specifically, the invention relates to an active suspension system which performs vehicular height regulation and vehicular attitude regulation and further provides modulate transition at a vehicular attitude influenced by acceleration at a critical magnitude.
2. Description of the Background Art
Japanese Patent First (unexamined) Publication (Tokkai) Showa 62-295714 and 63-62-235112 disclose typical structures of active suspension systems. The active suspension system disclosed in the former publication is designed to perform anti-rolling suspension control on the basis of lateral acceleration exerted on the vehicular body. The shown system adjusts fluid pressure working chambers in a hydraulic cylinder for adjusting damping force resisting against rolling moment and thereby suppressing vehicular rolling. On the other hand, the later publication discloses an active suspension system which performs anti-pitching control on the basis of longitudinal acceleration exerted on the vehicular body. The system also adjusts the fluid pressure in the working chambers in the hydraulic cylinders so as to suppress vehicular pitching motion. The later publication further discloses the use of variable gains for controlling front and rear suspension systems, with which gains the longitudinal acceleration indicative signal is amplified to derive front and rear suspension control commands. The variable gain provided in the later publication is expected to provide higher precision and optimum suspension control performance.
In such prior proposed active suspension systems, the fluid pressure in the working chamber is maintained at a predetermined neutral pressure as long as the lateral and/or longitudinal acceleration exerted on the vehicular body is maintained at zero. When vehicle rolls, the fluid pressure at left and right suspension systems are adjusted by left and right suspension control commands having equal values but having opposite phases or polarities for hardening suspension systems oriented outside of the curve and softening suspension oriented inside of the curve. Similarly, when vehicular pitching is caused, front and rear suspension control commands having the equal value but opposite phases are supplied to the front and rear suspension system, for suppressing vehicular pitching motion magnitude. When acceleration greater than a predetermined maximum is exerted on the vehicular body, the fluid pressure in the working chamber is adjusted to a predetermined maximum and minimum level and maintained thereat. The predetermined maximum acceleration may be set depending upon the capacity of the hydraulic cylinder, acceptable magnitude of vehicular attitude change, desired critical level of corning performance and so forth.
With such prior proposed active suspension system, vehicular attitude can be successfully regulated as long as the acceleration exerted on the vehicular body is maintained below the predetermined maximum level. On the other hand, when the acceleration becomes greater than maximum level, sudden change of vehicular attitude is caused to cause degradation of the riding comfort and driving stability.